Ground based radar is very effective for searching and tracking high flying targets. However, ground based radar is much less effective against very low flying targets because of the short distance to the horizon and because of objects such as trees, buildings, or hills that may mask the low flying targets. Those limitations on detection by radar are effectively exploited by the cruise missile whose precise control enables it to fly closer to the ground or water than a manned aircraft. An ideal solution for the detection of such low-flying objects is to place a radar high above the ground and look down. Airplanes, blimps, and stationary balloons can be used as carriers of such radars. The distance to the horizon then becomes very large and a wide area can be placed under observation.
The primary problem of the look-down radar is the clutter produced by the surface of the Earth. One must find a small target in front of the huge surface of the Earth. The range resolution of the radar can generally not be used to separate the target from the Earth's surface; an exception occurs where the target is almost exactly below the radar. A human observer uses two principles to recognize a target under these conditions:
(a) the velocity of the target relative to the surface of the Earth is recognized; PA1 (b) the shape or pattern of the target is recognized.
It is noteworthy that most animals utilize velocity information better than they do shape information; this is the reason why a dog can recognize a running rabbit much better than a motionless rabbit. We infer from this observation that recognition by the use of velocity requires less signal processing than recognition by the use of the shape.
Radar, at least since 1943, has utilized velocity information to discriminate between moving targets and stationary clutter. The methods developed for such radars are referred to as doppler processing and moving target indication (often abbreviated to "MTI"). These methods of velocity discrimination are of crucial importance because the difficulty in distinguishing a target from the clutter by look-down radar is severe and a pressing need exists for appreciable improvement. Here disclosed is a novel method for recognizing a target by its shape or pattern which can be combined with known methods of velocity discrimination to provide the two principal components needed for recognition of a moving target.